Teaching Case

A 47-year-old woman presented with a 1-month history of left flank pain, without gross hematuria. No history of other diseases or surgery. Physical examination and laboratory findings were unremarkable.

Unenhanced CT of the abdomen showed a 5.5 x 5.0 cm solid well-circumscribed mass in the left renal sinus , with soft-tissue attenuation, that stretched the dilated renal pelvis. Contrast-enhanced CT showed relatively homogeneous  attenuation of this mass but not invading the collecting system or renal parenchyma.

Solitary fibrous tumour (SFT) is a rare spindle cell neoplasm that is most often found in the pleura but is now recognized as a neoplasia arising in many parts of the body [1,2].

Wide surgical excision is the mainstay of treatment and adjuvant radiotherapy and chemotherapy are not required in routine cases [5]. Due to their rarity and lack of randomized control trials, there is no global consensus on treatment of SFTs [3].

Accurate diagnosis is essential for appropriate treatment and immunohistochemistry is the most sensitive and specific means of diagnosing SFT [3]. Typical immunohistochemical characteristic is a high positivity for CD34. As SFT has a malignant potential, careful follow-up is mandatory, searching for local recurrence or metastasis which was reported in few cases [6].

Solitary fibrous tumor of the kidney is a neoplasia believed to arise from primitive mesenchymal cells of the renal capsule, parenchyma or parapyelic connective tissue. SFTs of the kidney are extremely rare. Fat-forming SFT has been described and this fat-containing variant of SFT arising in the kidney was reported [2]. Most solitary fibrous tumors are benign; however, 10%–15% of extrapleural solitary fibrous tumours show malignant behavior in the form of recurrence or metastatic disease [5].

CT = SFT is usually a well-circumscribed lobulated soft-tissue mass in the region of the renal sinus or capsule with homogeneous enhancement. Necrosis, hemorrhage, and calcifications are rare [1,4,5].

MR = SFT is isointense to the renal cortex on T1-weighted images. On T2-weighted MR images, alternate areas of extremely low signal intensity and moderately high signal intensity distributed in a radial configuration [1,4,5].

CT, US and MRI features are not specific for the diagnosis of SFT and they are usually diagnosed as renal cell carcinoma preoperatively. The final diagnosis is always based on immunohistochemical study [6].

The possibility of SFT should be considered in the differential diagnosis for a well-circumscribed lobulated soft-tissue mass in the region of the renal sinus or capsule, with relatively homogeneous enhancement.

All patient data have been completely anonymised throughout the entire manuscript and related files.